Is there any recommended guide on how to cross-compile MPD? I don't know much about build systems and compilation -- I've just always run ./configure (and installed any dependencies it complained that I was missing until that succeeded) and then make, and it's gone OK. Obviously that's just for the local system.

What I don't want to do is pollute my workstation with a bunch of globally-installed dependencies, so I figure maybe a VM would be the answer, or perhaps there's some natively-running tool I'm unaware of which can partition a separate environment where all the necessary tools and dependencies can be.

One thing I'm worried about is how the dependencies work -- would I need the same versions of the dependencies on both machines? Or would I compile everything into a static chunk on the workstation? If that's a choice, what are the pros and cons each way?

There is nothing special with MPD regarding cross-compilation. MPD uses autotools, and autotools suports cross-compilation very well. The problems you need to solve are thus pretty generic. Collecting all the host dependencies on the build machine is, of course, the hardest part. You need compatible versions of those host libraries. Maybe the easiest way to do that is to have a copy of your Rpi filesystem (with all required -dev packages installed) on your build machine.